The present invention relates to an off-shore production platform, and especially relates to an off-shore oil production platform which includes an upper barge extending above the level of the sea and being connected to a completely submerged hollow lower base by partially submerged connecting legs forming a buoyancy tank and extending substantially vertically.
Platforms of this type are called semi-submersible platforms. In order to make such platforms stable during production, the lower base is ballasted (for example by filling it with seawater). In known platforms, the legs are formed by cylindrical columns with solid walls delimiting along their entire height a closed space forming a buoyancy tank for the platform.
These platforms do not rest directly on the sea bed and are simply anchored by mooring lines. They are thus very sensitive to swelling of the sea, which causes rising and falling vertical movements of the platform. The amplitude of these movements may reach high values. This phenomenon makes oil production from the platform difficult.
In order to attempt to provide a solution to this problem, it has been proposed to extend the length of the legs so that the base is submerged at a great depth. The result obtained by implementing this solution remains imperfect, and such platforms are complicated to manufacture and to install. Furthermore, they are temporarily unstable during installation.
French patent application FR-A-2,713,588 describes a jack-up platform including legs formed of a metal lattice along their entire height. Floats built into the legs allow the platform to be made buoyant. However, they are not intended to reduce the vertical movements of the platform.